1. Technical Field
The present invention relates to a method for manufacturing a nonvolatile semiconductor memory element which has a resistance value that changes according to application of a voltage pulse, and to the variable resistance nonvolatile semiconductor memory element.
2. Background Art
Recent years have seen increasing high performance in electronic devices such as mobile information devices and information appliances following the development of digital technology. With the increased high performance in these electronic devices, miniaturization and increase in speed of semiconductor elements used are rapidly advancing. Among these, application for large-capacity nonvolatile memories represented by a flash memory is rapidly expanding. In addition, as a next-generation new-type nonvolatile memory to replace the flash memory, research and development on a variable resistance nonvolatile semiconductor memory element which uses what is called a variable resistance element is advancing. Here, variable resistance element refers to an element having a property in which a resistance value reversibly changes according to electrical signals, and capable of storing information corresponding to the resistance value in a nonvolatile manner.
As an example of such variable resistance element, nonvolatile semiconductor memory elements, which are provided with variable resistance elements having variable resistance layers in which transition metal oxides of different oxygen content atomic percentage are stacked, are proposed. For example, Patent Literature 1 discloses selectively causing the occurrence of oxidation/reduction reaction in an electrode interface which is in contact with a variable resistance layer having a high oxygen content atomic percentage, to stabilize resistance change.
On the other hand, as an example of a large-capacity nonvolatile memory device incorporating the variable resistance elements, a cross point nonvolatile memory element has been proposed. For example, Patent Literature (PTL) 2 discloses an element having a configuration in which a variable resistance film is used as a memory unit and a diode element is used as a switching element. According to PTL 2, a variable resistance element includes a variable resistance layer for storing information according to a change in electric resistance in response to electric stress applied thereto, and a top electrode and a bottom electrode placed on either side of the variable resistance layer. Formed on the variable resistance element is a two-terminal nonlinear element having a nonlinear current-voltage characteristic for supplying a current bidirectionally. The memory cell is formed of a series circuit including the variable resistance element and the nonlinear element. The nonlinear element is a two-terminal element having a nonlinear current-voltage characteristic similar to that of a diode element or the like in which a current changes inconstantly with respect to a voltage change. Because current flows bidirectionally when rewriting the memory cell, a varistor (e.g., ZnO or SrTiO3) having a current-voltage characteristic which is bidirectionally symmetric and nonlinear is used as the nonlinear element. With the above configuration, it is possible to supply a current with a current density of 30 kA/cm2 or higher which is required for rewriting for the variable resistance element, and an increase in capacity can be realized.